Charge control system for trans-critical vapor cycle systems

Abstract

A cooling system includes a main refrigerant circuit that includes a compressor, a heat rejection heat exchanger, one of an expander and an expansion device, at least one evaporator coupled to a thermal load, and a suction accumulator. A charge management circuit includes a charge management receiver configured in parallel with the compressor and the heat rejection heat exchanger. A controller is configured to accumulate and discharge reserve refrigerant to and from the charge management receiver to provide flexibility in system operation as refrigerant in the main refrigerant circuit operates in sub-critical, trans-critical, and super-critical modes of operation.

What is claimed is: 
     
       1. A cooling system comprising:
 a main refrigerant circuit, comprising:
 a compressor; 
 a heat rejection heat exchanger; 
 one of an expander and an expansion device; 
 at least one evaporator coupled to a thermal load; and 
 a suction accumulator; 
 
 a charge management circuit having a charge management receiver configured in parallel with the compressor and the heat rejection heat exchanger, the charge management receiver having an inlet configured to receive a refrigerant from a discharge of the compressor, wherein a refrigerant pressure at the inlet is a result of a discharge pressure of the compressor after having passed through a refrigerant line between the discharge of the compressor and the inlet of the charge management device, and wherein the inlet is downstream of the heat rejection heat exchanger; and 
 a controller configured to accumulate and discharge reserve refrigerant to and from the main refrigerant circuit based on the discharge pressure of the compressor to provide flexibility in system operation as refrigerant in the main refrigerant circuit operates in sub-critical, trans-critical, and super-critical modes of operation. 
 
     
     
       2. The cooling system as claimed in  claim 1 , wherein the thermal load is from an aircraft and the refrigerant is CO 2 . 
     
     
       3. The cooling system as claimed in  claim 2 , further comprising:
 a first normally open receiver valve on a first side of the charge management receiver and a second normally open receiver valve on a second side of the receiver, the first side having a pressure that is higher than that of the second side. 
 
     
     
       4. The cooling system as claimed in  claim 3 , the main refrigerant circuit further comprising a hot gas bypass valve configured to divert hot gas from a discharge of the compressor to one of an inlet to the suction accumulator and a recuperative heat exchanger. 
     
     
       5. The cooling system as claimed in  claim 4 , wherein the controller is further configured to position the first and second normally open receiver valves in an OFF position when the hot gas bypass valve is positioned in an ON position. 
     
     
       6. The cooling system of  claim 3 , wherein if the discharge pressure is above an optimal pressure due to a reduction in an ambient temperature, the controller is configured to open the first receiver valve to move refrigerant from the main refrigerant circuit to the charge management circuit, and if the discharge pressure is below the optimal pressure due to an increase in the ambient temperature, the controller is configured to open the second receiver valve of the charge management receiver to move the refrigerant from the charge management circuit to the main refrigerant circuit. 
     
     
       7. The cooling system as claimed in  claim 1 , wherein the controller is configured to compare a suction pressure of the compressor to a pressure within the charge management receiver, and determine, based on the comparison, whether a charge of refrigerant within the main refrigerant circuit is sufficient or insufficient. 
     
     
       8. The cooling system as claimed in  claim 1 , wherein the system has “n” evaporators connected in parallel and at least “n−1” evaporators have back-pressure regulators located downstream from the related evaporators. 
     
     
       9. The cooling system as claimed in  claim 8 , wherein a hot gas bypass valve is configured to control evaporator capacity based on superheat values at the evaporator exits by comparing the superheat values to set points: if at least one superheat is below a related set low point and all other superheat values are within a range the hot gas bypass valve opens; if at least one superheat is above a related set high point and all other superheat values are within the range the hot gas bypass valve closes. 
     
     
       10. The cooling system as claimed in  claim 8 , wherein the controller is configured to control an evaporator capacity based on the superheat at an exit of the evaporator by comparing the superheat to set points such that: each back pressure regulator opens when the superheat is above a set high point and each back pressure regulator closes when the superheat is below a set low point. 
     
     
       11. A method of operating a cooling system, the method comprising:
 passing a refrigerant in a main refrigerant circuit, the main refrigerant circuit comprising a compressor, a heat rejection heat exchanger, one of an expansion device and an expander, at least one evaporator coupled to a thermal load, and a suction accumulator; 
 passing the refrigerant from the main refrigerant circuit to a charge management circuit and at a discharge pressure of the compressor, wherein a refrigerant pressure at an inlet of the charge management device is a result of the discharge pressure of the compressor after having passed through a refrigerant line between the discharge of the compressor and the inlet of the charge management device, and wherein the inlet is downstream of the heat rejection heat exchanger, the charge management circuit having a charge management receiver configured in parallel with the compressor and the heat rejection heat exchanger; and 
 accumulating and discharging reserve refrigerant to and from the main refrigerant circuit based on the discharge pressure of the compressor to provide flexibility in system operation as refrigerant in the main refrigerant circuit operates in sub-critical, trans-critical, and super-critical modes of operation. 
 
     
     
       12. The method of  claim 11 , further comprising receiving the thermal load from an aircraft, and operating first and second receiver valves, positioned at respective low pressure and high pressure sides of the charge management receiver, based on ambient conditions of the aircraft. 
     
     
       13. The method of  claim 12 , further comprising:
 comparing a suction pressure of the compressor to a pressure within the charge management receiver; and 
 determining, based on the comparison, whether a charge of refrigerant within the main refrigerant circuit is sufficient or insufficient. 
 
     
     
       14. The method of  claim 12 , wherein first and second charge management receiver valves are positioned at an inlet to and an exit from the charge management receiver, wherein the valves are normally closed;
 further comprising shutting off the system and automatically isolating the charge management receiver from the main refrigerant circuit using the normally closed valves. 
 
     
     
       15. The method of  claim 12 , wherein if the discharge pressure is above an optimal pressure due to a reduction in an ambient temperature, the method further comprises opening the first receiver valve to move refrigerant from the main refrigerant circuit to the charge management circuit, and if the discharge pressure is below the optimal pressure due to an increase in the ambient temperature, the method further comprises opening the second receiver valve of the charge management receiver to move the refrigerant from the charge management circuit to the main refrigerant circuit. 
     
     
       16. An aircraft having a thermal load and a cooling system, the cooling system comprising:
 a main refrigerant circuit that includes a compressor, a heat rejection heat exchanger, an expansion device, at least one evaporator coupled to the thermal load, and a suction accumulator; 
 a charge management circuit having a charge management receiver configured in parallel with the compressor and the heat rejection heat exchanger, the charge management receiver having an inlet configured to receive a refrigerant from a discharge of the compressor, wherein a refrigerant pressure at the inlet is a result of a discharge pressure of the compressor after having passed through a refrigerant line between the discharge of the compressor and the inlet of the charge management device, and wherein the inlet is downstream of the heat rejection heat exchanger; and 
 a controller configured to provide flexibility in system operation as refrigerant in the main refrigerant circuit operates in sub-critical, trans-critical, and super-critical modes of operation, based on the discharge pressure, to:
 store redundant refrigerant in the charge management receiver and from the main refrigerant circuit; and 
 discharge the redundant refrigerant from the charge management receiver and to the main refrigerant circuit. 
 
 
     
     
       17. The aircraft as claimed in  claim 16 , further comprising:
 first and second receiver valves on respective low and high pressure sides of the charge management receiver. 
 
     
     
       18. The aircraft as claimed in  claim 16 , wherein the controller is configured to compare a suction pressure of the compressor to a pressure within the charge management receiver, and determine, based on the comparison, whether a charge of refrigerant within the main refrigerant circuit is sufficient or insufficient. 
     
     
       19. The aircraft as claimed in  claim 16 , the charge management circuit further comprising valves positioned at the inlet to and an exit from the charge management receiver, wherein the valves are normally closed such that, when the system is off, charge management receiver is automatically isolated from the main refrigerant circuit. 
     
     
       20. The aircraft of  claim 17 , wherein if the discharge pressure is above an optimal pressure due to a reduction in an ambient temperature, the controller is configured to open the first receiver valve to move refrigerant from the main refrigerant circuit to the charge management circuit, and if the discharge pressure is below the optimal pressure due to an increase in the ambient temperature, the controller is configured to open the second receiver valve of the charge management receiver to move the refrigerant from the charge management circuit to the main refrigerant circuit.